A real-time regex explanation tree would be helpful for many reasons and different types of users.

For n00bs, they can enter a regex and read out an explanation of how it works in plain text, or print it out as documentation.

For intermediate to advanced users, it would provide a way for RegexBuddy to provide warnings about cross-browser inconsistency and other types of issues, as well as feedback about exactly why certain parts of their regex are highlighted as errors or in a way they did not expect (e.g. with the whole octal/backreference debacle).

My idea about the interface is that the subject text area could be split in two, with the regex field on top across the entire UI, the explanation tree on the left, and the subject text field on the right.

The tree could possibly be implemented using the Ext tree control, in which case it might be best to re-implement the entire site using Ext. Other UI solutions should be considered as well.